1. Field of the Invention
The present invention relates to a method and to apparatus for regulating the modulation current of laser diodes.
2. Description of the Prior Art
When using laser diodes as optical transmitters in light wave guide transmission systems, the operating point of these components must be kept constant vis-a-vis aging and temperature changes on the basis of a suitable regulation method. Since the characteristic of the laser diodes generally employed has a noticeable threshold, the regulation of the operating point of a laser diode is composed of regulating a preconduction current and of regulating the modulation current superimposed on the preconduction current in the characteristic region of the actual laser emission above the threshold. Both the threshold current of the laser diode and the slope of the characteristic above the threshold are subject to variation caused by aging and temperature and therefore require a corresponding readjustment of the preconduction current and the modulation current. For regulating the operating point, a portion of the generated light is usually converted into an electrical signal with a monitor photodiode that is optically coupled to the laser diode and this electrical signal is used for generating a control signal. In a less-involved regulation, the preconduction current is thereby adjusted via the evaluation of the average light output power and the modulation current is held constant while foregoing a regulation of the modulation current.
According to FIG. 1, in a method for regulating the operating point of laser diodes described by Smith in the publication Electronics Letters, Vol. 14, 1978, pp. 775-776, a relatively low-frequency pilot current PSI is superimposed on the modulation current MI, the pilot current PSI effecting a corresponding pilot signal portion PSL in the generated light. An accurate stabilization of the operating point of the preconduction current results with this method in the region of the threshold current, as does an adjustment of the light signal amplitude by regulating the modulation current on the basis of a combined evaluation of the average light output power as well as of the evaluation of the characteristic slope at the operating point.
Further developments of digital signal transmission have yielded useful digital signals having bit rates in the Gigabit/s region, whereby disturbance can result due to cut-in delays and relaxation resonance of the laser diode insofar as the preconduction current is set below the range of the threshold current. The known and proven method of relative evaluation of characteristic slope does not come into consideration for this characteristic operating range because the preconduction current must be set above the threshold current range.
One possibility for regulating laser transmitters for useful signals in the Gigabit/s range is the regulation of only the preconduction current given a constant modulation current or in combination with a simplified regulation of the modulation current. Difficulties can thereby result, however, in that the characteristic slope is changed due to an aging of the laser diode and non-linear changes in characteristics can also occur. A direct evaluation of the light signal amplitude of the digital useful signal is, in fact, conceivable, but requires a comparatively-high expense, particularly for the regulation of the modulation current, because of the high-frequency bandwidth of the useful digital signal, whereas the regulation of the preconduction current can occur with known methods.